Remove core dependency on encoding.json

Rationale: Reduce footprint.

Also see #13.

Right now, the `encoding.json` dependencies are minimal. The most important use is for `Spec.PatternSyntax` and related code, which knows how to parse branch patterns rendered in JSON.

Approach: Support pluggable pattern parsers.

Start by tracing the core uses of encoding.json, especially Spec.PatternSyntax and the related branch-pattern parsing mentioned in the issue. Read issue #13 for context and map the current dependency boundary. Done means pattern parsers are pluggable and the core no longer depends on encoding.json.

Written by the indexing model from the issue text.